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Hardware
Over its lifetime, the Genesis came in three different models, each of which had several motherboard revisions. For more details, see Sega Genesis Hardware.
Model 1
Model 1 Genesis
The model 1 Sega Genesis is the largest model and features a headphone jack with a volume slider. There are at least seven motherboard revisions of the model 1 that are known to exist, labeled VA1 - VA7. Early revisions, which bear the text "HIGH DEFINITION GRAPHICS" in white above the cartridge slot, are considered to have the best audio output of any version of the Genesis. The VA7 is the latest revision, and internally they are similar to the model 2 Genesis, and they do not have the "HIGH DEFINITION GRAPHICS" text on them.
Model 2
Model 2 Genesis
The model 2 was smaller than the model 1 and lacked a headphone jack but was capable of outputting stereo audio through its rear A/V multi-out port.
Model 3
Model 3 Genesis
The model 3 suffered from compatibility issues.